A cholinergic antagonist, mecamylamine, blocks the phase-shifting effects of light on the circadian rhythm of locomotor activity in the golden hamster

Despite the well known role of the light-dark cycle in the entrainment of circadian rhythms, very little is known about the neurochemical events that mediate the effects of light on the mammalian circadian clock. Recent anatomical and pharmacological data support the hypothesis that acetylcholine may be involved in relaying light-dark information from the retina to, or within, the circadian clock of rodents. If acetylcholine is required for this response, it should be possible to block the phase-shifting effects of a light pulse by blocking cholinergic neurotransmission. To test this possibility, hamsters free-running in constant darkness received an intraventricular injection of the anticholinergic drug, mecamylamine (450 μg), 10 min before being exposed to a 5-min pulse of light known to induce sub-maximal phase shifts in the circadian rhythm of wheel-running behavior. Compared to vehicle-injected control animals, mecamylamine treatment blocked or reduced both the phase-advancing and phase-delaying effects of light. These results support the hypothesis that acetylcholine is involved in mediating the phase-shifting effects of light on the mammalian circadian clock.